In the helicopter art, blade tip sweep or taper, or combinations of both have been used to improve helicopter forward flight performance. To reduce tip lift, however, this prior art utilized a substantial amount of twist in the blade tip and had to pay the penalty of increased drag resulting in increased rotor drive power requirements in forward flight due to the negative pitch or angle of attack on the advancing blade. Typical examples of the prior art using swept or tapered tips to increase helicopter forward flight performance and utilizing twist to unload the tip and control tip trailing edge vortex are Jepson U.S. Pat. No. 3,822,105 and pending Fradenburgh U.S. Application Ser. No. 968,595 entitled "Improved Helicopter Blade" and filed Dec. 11, 1978. Monteleone U.S. Pat. No. 3,721,507 teaches sweep and tip thinning but not anhedral, nor is Monteleone concerned with hover.
Lowson et al U.S. Pat. No. 4,077,741 utilizes a helicopter blade with a swept tip but for the purpose of controlling a tip leading edge vortex, as opposed to the trailing edge tip vortex of interest herein, so as to cause the tip to act as a conventional Delta wing.
Certain prior art patents teach anhedral blade or wing tips, but not in combination with sweep and taper as taught herein to improve helicopter hover performance. Sargent U.S. Pat. No. 3,411,738 teaches an anhedral tip but specifically teaches that the tip span should be small in comparison to the tip chord, which is in direct opposition to our teaching. Also, the Sargent patent teaches an anhedral configuration whereby the anhedral span diminishes from trailing edge to leading edge. Our invention incorporates an anhedral span which is constant along the local chord line. de la Cierva U.S. Pat. No. 1,692,081 teaches droop or tip anhedral but to improve the dynamic behavior of freely rotating autogiro wings. de al Cierva uses the drooped tips to provide the blade rotational centrifugal forces necessary to make the hinged, autogiro rotor stable. Of course, since the autogiro utilizes freely rotating wings it does not offer the hover operation of the power driven helicopter rotor to which our invention is directed.
Giles U.S. Pat. No. 3,399,731 teaches swept helicopter rotor wings to improve forward flight operation but includes no anhedral and is not concerned with hover performance improvement.